The objectives of this contract are: l) to continue studies to define the contribution of specific subpopulations of CD34+ marrow cells to early engraftment and long term stable reconstitution of lymphohematopoiesis in vivo in lethally irradiated baboons; 2) to define the kinetics with which these purified subpopulations of CD34+ cells engraft; 3) to define the cell types and function of cells derived from the transplanted purified subpopulations of CD34+ marrow cells; 4) to determine the contribution of purified CD34+ cells and subpopulations of CD34+ cells that circulate in response to specific growth factors to early engraftment and long term stable reconstitution of lymphohematopoiesis in vivo in lethally irradiated baboons; 5) to define the kinetics with which these purified cells engraft; 6) to define the cell types and function of cells derived from the transplanted purified subpopulations of CD34+ blood cells; and 7) to determine if purified subpopulations of CD34+ cells from blood or marrow, that contain only long term marrow repopulating cells and are depleted of cells responsible for early engraftment, the kinetics of engraftment can be enhanced by treatment with specific hematopoietic growth factors either in vitro or in vivo.